[time-nuts] Re: MHM-A1 maser temperature stabilization

[Bob Camp]

Hi

I’ve looked at Peltier gizmos. If you are pumping 100W of heat they seem to use
quite a bit of energy to get the job done. A compressor based setup of some sort
might do a better job. With any sort of cooler, my fear is a loss of cooling. If the
device is very well insulated, damage could occur. 

The 120 L bucket of water is about half the size of my “rough guess” thermal mass.
I fear that the guess may be way low ….

Bob

> On Jan 17, 2023, at 2:09 PM, Neil Smith via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 
> One thing I've tried to reduce diurnal and seasonal thermal variation is putting the sensitive components in a hyper-insulated enclosure with high thermal mass mounted inside another stabilised oven, also with plenty of thermal mass, but then connecting a well insulated copper bar from the inner enclosure to a stabilised Peltier cooling plate mounted outside. That gives me a very stable thermal flow with a large temperature gradient so the inside oven thermal control loop can stay in a nice linear regime and doesn't have to deal with large changes in required heating power.
> It's overkill for my application, which is a only a 10MHz Morion MV-89a OCXO. 
> 
> I also have a 120 litre water tank with a circulating pump that can keep a cooling plate at a very stable temperature over periods of an hour or so, but its main use-case is for cooling high powered RF amplifiers. 
> 
> Neil
> https://youtube.com/MachiningandMicrowaves
> 
>> On 17 Jan 2023, at 18:05, Bob Camp via time-nuts <time-nuts at lists.febo.com> wrote:
>> 
>> Hi
>> 
>> I may well be missing something obvious in the electronics to heat conversion process
>> ( it certainly would not be the first time … :). That said:
>> 
>> I set up a box with foam and the 100W from the device inside heats it up by about
>> 4 C. You immediately get into questions about “4C where?”.  Just accept the number and
>> move on for now. 
>> 
>> The room wanders a bit. How much depends on a lot of things. 2C is not a bad guess on
>> most days. At the wrong time of year it could be 2X that. There is a day to night component 
>> that usually dominates. 
>> 
>> Target is to damp out the 24 hour swing. The R/C should be longer than that. How much
>> will depend a bit on the temp stability target. 1C/24 hrs is an improvement. The folks who 
>> made the gizmo suggest that 0.1C is a better target. Yes, the “C” in the R/C just moved 10:1
>> as you changed that target. (There are other issues as well, for now, let’s ignore them)
>> 
>> As I do this in my usual hand waving fashion, I come up with hundreds of liters of water for
>> the thermal mass. It just goes up if I move from 1C and get closer to 0.1C. 
>> 
>> This assumes that everything else is zero mass. Things do “follow” the room temp with an 
>> lag of a couple hours. Even to get to 1C, significant thermal mass needs to be added.  ( or 
>> lots more insulation, stick with mass for now ). 
>> 
>> Am I missing something or is the “hundreds of liters” guess more or less in the right range?
>> There are some practical implications to playing with groups of ten  “jerry cans” full of water.
>> (even as a “try it and see" experiment). 
>> 
>> No, this isn’t an attempt to come up with a full up answer. It’s just a question about what 
>> the rough order of magnitude is in this approximate case. There are enough holes in the
>> data above that any sort of precise answer is data limited. 
>> 
>> Bob
>> 
>>> On Jan 17, 2023, at 4:07 AM, Poul-Henning Kamp via time-nuts <time-nuts at lists.febo.com> wrote:
>>> 
>>> --------
>>> Skip Withrow via time-nuts writes:
>>> 
>>>> One issue that I have been up against is the temperature variations of the
>>>> lab location.  The diurnal excursions of the oven heaters were clearly
>>>> visible.  My solution was to build an environmental box (1.5" foam
>>>> insulation board) around the unit 
>>> 
>>> Apologies for harping about this again:
>>> 
>>> Please think in terms of thermal impedance!
>>> 
>>> Look at it as electronics for a second:
>>> 
>>> You have put a (very big!) resistor between your AC noise source
>>> (the heating) and the sensitive kit (the maser).
>>> 
>>> Because your resistor is so large, you now have an over-temperature
>>> problem inside your enclosure, which you have tried to mitigate with
>>> a fan.
>>> 
>>> What you actually need is a low-pass filter with a cut-off lower than 1/24h.
>>> 
>>> That means /some/ insulation, but not so much that your maser cannot
>>> get rid of the heat it produces, and /a lot/ of thermal mass on the
>>> inside to "short the AC to ground".
>>> 
>>> This is not magic, and the math is trivial when you already know electronics.
>>> 
>>> -- 
>>> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
>>> phk at FreeBSD.ORG         | TCP/IP since RFC 956
>>> FreeBSD committer       | BSD since 4.3-tahoe    
>>> Never attribute to malice what can adequately be explained by incompetence.
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